This paper discusses the power density and temperature induced issues in modern on-chip systems due to the high integration density and roadblock on the voltage scaling. First, the emerging dark silicon problem is discussed, and the corresponding critical research challenges in future chips are enumerated. Afterwards, we present an overview of some key research efforts and concepts that leverage dark silicon for performance and reliability optimization of on-chip systems under power or temperature constraints. The summarized works account for heat transfer inside a chip, as well as the varying performance and power trade-offs of gray silicon, that is, the potential benefits of operating at lower-than-nominal voltage and frequency levels. Besides realizing reliability-heterogeneous architectures, reliability of an on-chip system is enhanced by exploiting dark silicon for aging deceleration and resilience-driven resource management to mitigate soft-errors. Several of the tools discussed in this paper are available for download at http://ces.itec.kit.edu/download.